Portable devices with image sensor (e.g. digital cameras, mobile communication terminals, etc.) have recently been developed and are currently in the market. Image sensor comprises an array of tiny photosensitive diodes known as pixels or photosites. The pixels themselves usually do not extract color from light, but convert photons into electrons from a wide spectrum band. To record color images with a single sensor, the sensor is filtered so that different pixels receive lights with different colors. This type of sensor is known as a color filter array, or CFA. Different color filters are disposed across the sensor in a predefined pattern.
The most common pattern is the Bayer color filter array developed by Kodak. The CFA of a color image generally follows a Bayer pattern. In other words, a half of the total number of pixels is green (G), and two quarters of the total number are assigned red (R) and blue (B). To obtain color information, the color image pixels are formed as a repetitive pattern of red, green, or blue filters, 2×2 arrays in the case of a Bayer pattern, for example.
The Bayer pattern is based on the premise that the human eye derives most of the luminance data from the green content of a scene. Therefore, an image with a higher resolution may be generated when more of the pixels are made to be green, compared to when an equal number of red, green, and blue pixels alternate.
However, the conventional image sensor on recently marketed portable devices had a problem of image distortion due to the geometric pattern of the pixel array composition. This is because of its small outer lens and high f number.
FIG. 1 is a diagram illustrating the disparity in transmissivity, and FIG. 2 is a graph illustrating the change in signal amplitude according to the distance from the center.
As illustrated in FIG. 1, the pixels in the central part of the image sensor and the pixels in the outer perimeter are exposed to the light source from different positions. These minute differences in position cause differences in illumination, and differences in illumination affect color because of differences in light frequency and refractive ratio.
Consequently, color distortion and reduction in signal amplitude dependant on the position of the pixels inevitably occur, which cause degradation in the quality of original images.